This invention concerns cemented carbide rolls for hot-forming steel rod in multi-stand rolling mills, especially in a finished rod diameter range of 7/32 inches to 1/2 inch. Carbide rolls, operating at rod temperatures typically in the 1700.degree. F. to 2200.degree. F. range, have gained wide use in multi-stand steel-rod rolling mills and, to a large extent, have replaced chilled cast iron rolls, especially in finishing roll mill stands.
The development of twist free rod mills allowed the use of higher, more economical hot rolling speeds without sacrifice of rod product dimensions or rod surface condition. The successful introduction of cemented carbide rolls of homogeneous, single-composition, tungsten carbide-cobalt alloys provided a roll material capable of being designed to withstand higher rolling speeds.
The sole hard carbide constituent in these roll alloys preferred by those skilled in the art, and most successful in application, has been tungsten carbide (WC) and cobalt.
The realization of the benefits of still greater rolling speeds of which improved mill design is now capable, however, requires roll materials possessing more toughness. Both the surface degradation of roll groove surfaces, or other working surface configuration, and massive roll fracture are related to several factors, among which major factors are thermal cracking caused by alternate heating and cooling of the mill roll as it encounters the hot steel rod and stresses due to mounting and torque transmission.
Rolls used for slower rolling speeds and larger rolling diameters, such as pre-finishing mills and bar mills having a finished rod diameter of one-half inch to three inches, are subject to even greater thermal stress because thermal cycling is accelerated by longer time intervals of roll-to-work contact and cooling exposure and, also, higher rolling torque and stresses due to the slower speed.
One of the causes of mill roll failure is due to the tensile stress imposed on the inside diameter of the mill roll when mounted in working position. The rolls are usually mounted on mandrels with means for exerting radially outward contact with the inside diameter of the roll. The contact with the mandrel must be sufficient so as to effect torque transmission between the mandrel and the roll. As cemented carbides are usually relatively weak in tensile strength, the tensile force imposed by the mandrel can cause failure of the roll. Bending stresses due to high torque transmission can also contribute to roll failures.
The addition of tantalum carbide (TaC) to the outer part of the roll helps control the thermal cracking of the outer layer due to thermal stresses but increases the cost of manufacture of the mill roll. In order to reduce the overall cost of the mill roll, it was thought that the substitution of nickel (Ni) for some of the cobalt in the inner ring could be achieved without any major loss in tensile strength or wear resistance, (Ni) being a less expensive commodity than cobalt (Co). Surprisingly, the addition of Ni to the binder material of the inner ring dramatically increases the tensile strength rather than reducing it.
It is an object of this invention to provide a cemented carbide roll for hot forming steel rod in multi-stand rod or bar mills which is significantly more resistant to thermal cracking and stress related failures.
It is an additional object of this invention to provide a roll which possesses greater toughness as evidenced by longer roll service time and greater steel tonnage rolled before failure of roll would occur.